A conventionally used conductive paste for inner electrodes of laminated condensers, conductive patterns of circuit boards, and electronic parts such as electrodes and circuits of substrates for solar cells and display panels is a conductive paste produced by adding a silver powder to an organic vehicle, followed by kneading. A silver powder for such a conductive paste has been required to have, for example, an appropriately small particle diameter and a uniform particle size in order to respond to, for example, downsizing of electronic parts, and higher densification and finer lines of conductive patterns.
In the case where a highly water-soluble organic solvent or water is used as a solvent for the conductive paste, if a silver powder used for the conductive paste is not compatible to the organic solvent or water, the silver powder becomes in the form of a mass without being dispersed in the conductive paste. As a result, when the conductive paste is coated on, for example, a substrate, a film formed of the conductive paste will be uneven in thickness. This raises a problem with deterioration in conductivity and adhesion strength of the conductor formed by firing the conductive paste.
In order to solve the problem, there is proposed a method including: adding an alkali or a complexing agent to a silver salt-containing aqueous solution to form a silver oxide-containing slurry or a silver complex salt-containing aqueous solution; adding a reducing agent thereto to reduce and deposit silver particles; and adding a dispersing agent to the silver-containing slurry solution, the dispersing agent being a fatty acid, a fatty acid salt, a surfactant, an organic metal, a protective colloid, and at least one kind of a chelate-forming agent selected from a compound having an azole structure, dicarboxylic acid, oxycarboxylic acid, and salts thereof (see, for example, PTLs 1 and 2).
Also, there is proposed a method for producing a silver powder by adding a reducing agent to an aqueous reaction system containing silver ions to reduce and deposit silver particles, wherein two or more kinds of dispersing agents are added to a slurry reaction system before, after, or during reduction and deposition of silver particles, the dispersing agents being preferably, for example, a hydrophobic dispersing agent such as benzotriazole, stearic acid, or oleic acid and a hydrophilic dispersing agent such as gelatin or collagen peptide (see, for example, PTL 3).
In these proposals, however, when a solvent used for the conductive paste has high water solubility, hydrophilicity becomes insufficient depending on the kind of a dispersing agent attached onto the silver powder surface. Thus, compatibility between the silver powder and the solvent may become insufficient, resulting in a problem that the silver powder cannot be dispersed in the conductive paste. Also, depending on the kind of a surfactant and the like added, dispersing effects may be insufficient or absent based on the pH of the reaction solution and the coexisting ions. The kind of the surfactant usable is limited and also it is difficult to strictly control an amount thereof covering the silver powder surface.
In order to solve such problems, there is proposed a method in which after filtrating and water-washing the reduced and deposited silver powder, a phosphate ester-based surfactant is added when a wet cake having a water content of 20% to 80% is crushed in a mixer without being dried, and the resultant is again filtrated and washed with water, followed by drying, to remove the reaction solution and attach the surfactant (see PTL 1).
Also, there is proposed a method in which vacuum freeze drying is performed on a dispersion liquid obtained by dispersing silver particles in a solvent together with an alkylamine-based or alkylamine salt-based surfactant or a phosphate ester-based surfactant having a phosphorus content of 0.5% by mass to 10% by mass, to adsorb the surfactant onto the surfaces of the silver particles, to thereby produce a silver powder surface-treated with the surfactant (see, for example, PTL 4).
These proposed methods can produce a silver powder having a certain degree of hydrophilicity. However, problematically, an additional step is needed in the method for producing a silver powder including adding an alkali or a complexing agent to a silver salt-containing aqueous solution to form a silver oxide-containing slurry or a silver complex-containing aqueous solution, and adding a reducing agent thereto to reduce and deposit silver particles, followed by drying.